1. Field of the Invention
The present invention relates to a leak diagnosis system for an evaporative emission control system applied to an internal combustion engine.
2. Description of the Prior Art
A variety of evaporative emission control systems for automotive vehicles have been proposed and practically used. A typical evaporative emission control system is provided with a charcoal canister for preventing evaporative fuel in a fuel tank from being purged into the atmosphere. In this evaporative emission control system the charcoal canister adsorbs evaporative fuel from the fuel tank and discharges it into the engine by utilizing the negative pressure generated by an internal combustion engine.
However, such an emission control system has a possibility that evaporative fuel is leaked into the atmosphere if a leakage hole is generated at an evaporative fuel conduit or if an unsealed portion is generated at a connecting portion of the evaporative fuel conduit. If such a defect is generated in the evaporative emission control system, it is impossible to effectively operate the evaporative emission control system. In order to prevent such troubles, EPA (Environmental Protection Agency) and CARB (California Air Resources Board) have proposed a leakage diagnosis system and a practical method thereof. Such a leak diagnosis system is shown in FIG. 24 and a flowchart for controlling the diagnosis system is shown in FIG. 25. As shown in FIG. 24, an end of an evaporative fuel conduit 102a is connected to a fuel tank 101, and the other end of the evaporative fuel conduit 102a is connected to a charcoal canister 103 adsorbing evaporative fuel. A pressure sensor 106 is disposed in the way of the evaporative fuel conduit 102a in order to detect a pressure in the evaporative fuel conduit 102a. The signal from the pressure sensor 106 is inputted into an engine control unit 150 such that a changing speed of the pressure in the evaporative fuel conduit 102a is obtained. The engine control unit 150 has previously stored a decision value for diagnosing a leakage of evaporative fuel and implements the diagnosis by comparing the pressure changing speed with the decision value. An end of an evaporative fuel conduit 102b is connected to the charcoal canister 103, and the other end of the evaporative fuel conduit 102b is connected to an intake side of the internal combustion engine. A purge-cut valve 104 is disposed in the way of the evaporative fuel conduit 102b. The evaporative fuel adsorbed in the charcoal canister 103 is purged into the internal combustion engine during an engine operating condition upon opening the purge-cut valve 104. A drain-cut valve 105 is connected to a bottom of the charcoal canister 103 so as to cut the communication between the charcoal canister 103 and the atmosphere during the leak diagnosis.
Such a conventional leak diagnosis system implements the leak diagnosis according to the flow chart of FIG. 25 as follows.
In a step S10 the purge-cut valve 104 is opened during a predetermined engine operating condition, so that the evaporative fuel adsorbed in the charcoal canister 103 is purged into the engine due to intake negative pressure generated by the engine.
In a step S20 the drain-cut valve 105 is closed such that the negative pressure of the engine is applied to the evaporative emission control system which includes the fuel tank 101 and the evaporative fuel conduits 102a and 102b.
In a step S30 it is judged whether the pressure in the evaporative emission control system has reached a predetermined pressure value or not. Otherwise, in the step S30 it is judged whether a predetermined time has elapsed from the start of the sucking operation. When the judgment in the step S30 is "NO", the program returns to the step S20. When the judgment in the step S30 is "YES", the program proceeds to a step S40.
In the step S40 the purge-cut valve 4 is closed.
In a step S50 the engine control unit 150 monitors the change of the pressure in the inner space on the basis of the signal of the pressure sensor 106 and detects the pressure changing speed or pressure buildup speed toward the atmospheric pressure.
In a step S60 it is judged whether the detected pressure changing speed toward the atmospheric pressure is larger than the decision value or not. When the judgment in the step S60 is "YES", the program proceeds to a step S70 wherein it is judged that the system is in a leak condition. When the judgment in the step S60 is "NO", the program proceeds to a step S80 wherein it is judged that the system is in a no-leak condition.
Following the operation of the step S70 or S80, the program proceeds to a step S90 wherein the drain-cut valve 105 is opened.
However, since the above-mentioned leak diagnosis system is arranged to implement the leak diagnosis by obtaining the pressure changing speed toward the atmospheric pressure, after the pressure in the evaporative emission control system reaches a predetermined pressure by closing the drain-cut valve 105 and the purge-cut valve 104 is then closed. Accordingly, the conventional leak diagnosis system has some problems, such that it takes a long time for the implement of the leak diagnosis, and the conventional leak diagnosis system is complicated due to its complicated control algorithm. Furthermore, the pressure changing speed takes various values due to the fuel condition in the evaporative emission control system, such as a change of the inner space volume which changes according to the remaining amount of fuel in the fuel tank, or a change of the generating amount of fuel vapor which changes according to the fuel temperature, as shown in FIG. 20. Therefore, it is difficult to accurately diagnose the leak condition of the evaporative emission control system by using the conventional leak diagnosis system.